Optical semiconductor element, method of controlling the same and method of manufacturing the same

ABSTRACT

An optical semiconductor element includes a ring modulator, and a light absorbing material provided at a position apart from a path for a modulated light which is guided by the ring modulator, the light absorbing material absorbing a light leaked out of a ring waveguide of the ring modulator, and increasing a temperature of the ring waveguide.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/205,756, filed Mar. 12, 2014, which is a continuation application ofInternational Application PCT/JP2011/072814, filed Oct. 3, 2011, theentire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are directed to an opticalsemiconductor element, a method of controlling the same, and a method ofmanufacturing the same.

BACKGROUND

Practical application of optical devices using silicon as a material ofan optical waveguide is important to enable small sizing, largecapacity, and low power consumption of optical transmitting andreceiving devices. This is because an optical waveguide whose refractiveindex difference is large can be used, and therefore, it is advantageousfor small sizing compared to other materials. Besides, integration withan electronic circuit is easy, and therefore, it is possible tointegrate a number of optical transmitting and receiving devices on onechip. Particularly, characteristics of a modulator among optical deviceslargely affect on the power consumption and size of optical transmittingand receiving devices. In particular, a ring modulator among modulatorsis advantageous to enable the small sizing and the low power consumptionbecause an element in itself is small, a modulation voltage is small,and an optical loss thereof is small.

However, in a ring modulator, a wavelength band and a modulationefficiency are in a relationship of tradeoff. Accordingly, when highmodulation efficiency is to be obtained, the wavelength band is narrow,and it is difficult to match a wavelength of an incident light such as aCW (continuous wave) light and a resonant wavelength. Arts objecting tosolve the above-stated problems have been proposed, but it is difficultto enable a stable control.

-   Patent Literature 1: U.S. Patent Application Laid-open No.    2009/0169149-   Patent Literature 2: Japanese Laid-open Patent Publication No.    2009-200091

SUMMARY

According to an aspect of the embodiments, an optical semiconductorelement includes: a ring modulator; and a light absorbing materialprovided at a position apart from a path for a modulated light which isguided by the ring modulator, the light absorbing material absorbing alight leaked out of a ring waveguide of the ring modulator, andincreasing a temperature of the ring waveguide.

According to another aspect of the embodiments, a method of controllingan optical semiconductor element includes: heating an opticalsemiconductor element to a particular temperature or more with a heater,the optical semiconductor element including: a ring modulator; and alight absorbing material provided at a position apart from a path for amodulated light which is guided by the ring modulator, the lightabsorbing material absorbing a light leaked out of a ring waveguide ofthe ring modulator, and increasing a temperature of the ring waveguide;starting incidence of a modulated light to the ring modulator; and afterthe starting, stopping the heating with the heater. A heating value in afirst relationship is larger than a heating value in a secondrelationship at a ring resonant wavelength giving a maximum to the firstrelationship, the first relationship being a relationship between a ringresonant wavelength and a heating value according to absorption of aresonance light in the ring waveguide, and the second relationship beinga relationship between a heating value and a ring resonant wavelengthwhich changes according to the heating value in the ring waveguide. Theparticular temperature is a temperature corresponding to a nearestintersection on a short wavelength side from the ring resonantwavelength giving the maximum to the first relationship amongintersections between a graphic chart representing the firstrelationship and a graphic chart representing the second relationship.

According to still another aspect of the embodiments, a method ofmanufacturing an optical semiconductor element includes: forming a ringmodulator; and forming a light absorbing material at a position apartfrom a path for a modulated light which is guided by the ring modulator,the light absorbing material absorbing a light leaked out of a ringwaveguide of the ring modulator, and increasing a temperature of thering waveguide.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a view illustrating an example of a ring modulator;

FIG. 1B is a view illustrating characteristics of the ring modulatorillustrated in FIG. 1A;

FIG. 2A is a view illustrating another example of a ring modulator;

FIG. 2B is a view illustrating characteristics of the ring modulatorillustrated in FIG. 2A;

FIG. 3A is a view illustrating still another example of a ringmodulator;

FIG. 3B is a view illustrating characteristics of the ring modulatorillustrated in FIG. 3A;

FIG. 4A is a view illustrating a relationship between a heating valueand a ring resonant wavelength in a ring waveguide;

FIG. 4B is a view illustrating a relationship between a heating valueaccording to absorption of a resonance light and a ring resonantwavelength;

FIG. 4C is a view in which a graphic chart illustrated in FIG. 4A and agraphic chart illustrated in FIG. 4B are overlaid;

FIG. 5A is a view illustrating characteristics at an initial stage;

FIG. 5B is a view illustrating characteristics after heating;

FIG. 5C is a view illustrating characteristics after heating is stopped;

FIG. 6 is a view illustrating a relationship between a modulationvoltage V and a transmittance;

FIG. 7A is a view illustrating a relationship between a ring resonantwavelength and a heating value in consideration of modulation;

FIG. 7B is a view in which a graphic chart illustrated in FIG. 7A and agraphic chart illustrated in FIG. 4B are overlaid;

FIG. 8 is a view illustrating a relationship between a graphic chartillustrated in FIG. 6 and a locked wavelength;

FIG. 9 is a view illustrating operations for a burst signal in which aburst-off state and a burst-on state are mixed;

FIG. 10A is a view illustrating a layout of an optical semiconductorelement according to a second embodiment;

FIG. 10B is a sectional view along a I-II line in FIG. 10A;

FIG. 10C is a sectional view along a III-II line in FIG. 10A;

FIG. 11A is a view illustrating a layout of an optical semiconductorelement according to a third embodiment;

FIG. 11B is a sectional view along a I-II line in FIG. 11A;

FIG. 11C is a sectional view along a III-II line in FIG. 11A;

FIG. 12A is a view illustrating a layout of an optical semiconductorelement according to a fourth embodiment;

FIG. 12B is a sectional view along a I-II line in FIG. 12A;

FIG. 12C is a sectional view along a III-II line in FIG. 12A;

FIG. 13A is a view illustrating a layout of an optical semiconductorelement according to a fifth embodiment;

FIG. 13B is a sectional view along a I-II line in FIG. 13A;

FIG. 13C is a sectional view along a III-II line in FIG. 13A;

FIG. 14A is a flowchart illustrating an example of a method ofcontrolling an optical semiconductor element according to a sixthembodiment;

FIG. 14B is a flowchart illustrating another example of a method ofcontrolling an optical semiconductor element according to the sixthembodiment;

FIG. 15A to FIG. 15P are sectional views illustrating a method ofmanufacturing an optical semiconductor element according to a seventhembodiment in process sequence.

FIG. 16A is a view illustrating a structure of an optical semiconductorelement according to an eighth embodiment;

FIG. 16B is a view illustrating operations of the optical semiconductorelement according to the eighth embodiment; and

FIG. 17 is a view illustrating an optical semiconductor elementaccording to a ninth embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments will be explained with reference toaccompanying drawings.

First Embodiment

First, a first embodiment is described. FIG. 1A is a view illustratingan example of a ring modulator. Two straight waveguides 1, 2 and a ringwaveguide 3 between them are included in the ring modulator. Modulatingelectrodes 4 are provided at outside of the ring waveguide 3, and amodulating electrode 5 is provided at inside of the ring waveguide 3.

A CW light incident on an input port 6 of the waveguide 1 is guided toan output port 8 of the waveguide 2 when a ring resonant wavelength ofthe ring waveguide 3 and a wavelength of the CW light are matched. Thering resonant wavelength depends on the circumferential optical pathlength, and is a specified fraction of an integer of the circumferentialoptical path length. A CW light incident on the input port 6 of thewaveguide 1 is guided to an output port 7 of the waveguide 1 when a ringresonant wavelength of the ring waveguide 3 and a wavelength of the CWlight are not matched. A ring resonant wavelength of the ring waveguide3 changes, if a modulation voltage V applied on the ring waveguide 3 ischanged, a refractive index of the ring waveguide 3 is changed, and acircumferential optical path length is changed. Accordingly, atransmittance changes if a wavelength is fixed to a specific one, andtherefore, that may be used for a light intensity modulation. Forexample, in a case where a CW light having a wavelength illustrated inFIG. 1B is incident, a power of an output light to the output port 7 islarge when the modulation voltage V is a voltage V_(low), and is smallwhen the modulation voltage V is a voltage V_(high). On the other hand,a power of an output light to the output port 8 is small when themodulation voltage V is the voltage V_(low), and is large when themodulation voltage V is the voltage V_(high). Accordingly, it ispossible to obtain signals which have been modulated of light intensityfrom the output ports 7 and 8 by changing the modulation voltage Vbetween the voltage V_(high) and the voltage V_(low).

Next, a loss of an incident light is described. Here, it is assumed thatthe waveguide 2 is not provided as illustrated in FIG. 2A. An incidentlight circulates in the ring waveguide 3 when a wavelength of theincident light is close to the ring resonant wavelength, and suffers aloss during the circulation in the ring modulator. Accordingly, a power11 of a transmission light to an output port is small compared to a casewhen the wavelength of the incident light is far from the ring resonantwavelength, as illustrated in FIG. 2B. There are two kinds in the lossessuffered at this time. One of them is a loss caused by being scatteredby roughness of a side surface of the ring waveguide 3 and beingradiated because the incident light does not turn at a ring curvature.Namely, it is a loss of a power 13 of the light leaked out of the ringwaveguide 3. The other one is a loss caused by light absorption by amaterial of the ring waveguide 3. Namely, it is a loss of a power 12 ofthe absorbed light. The power 12 of the absorbed light is converted intoheat energy, and the ring waveguide 3 generates heat.

In the first embodiment, as illustrated in FIG. 3A, a light absorbingmaterial 9 is provided at a position apart from a path of the lightwave-guiding the ring modulator so as to effectively utilize a lightleaked out of the ring waveguide 3. The light absorbing material 9absorbs the light leaked out of the ring waveguide 3 and increases atemperature of the ring waveguide 3. The light absorbing material 9absorbs the light and converts a power 13 of the light into a power 14of heat. Then, the light absorbing material 9 increases the temperatureof the ring waveguide 3. As a result, as illustrated in FIG. 3B, theloss of the power of the leaked light is little, and a sum of the powers12 and 14 contributing to the absorption and heat generation is large.Namely, it is possible to convert energies of the scattered light andthe radiated light, which occupies a majority of the energy which islost when the light absorbing material 9 is not provided, into a heatenergy, and to make it contribute to the increase of the temperature ofthe ring waveguide 3.

Next, an effect according to the increase of the temperature of the ringwaveguide 3 is described. In general, a ring resonant wavelength becomeslonger with an increase of a refractive index according to a temperatureincrease of a ring waveguide. Accordingly, there is a relationship asillustrated in FIG. 4A between a heating value and a ring resonantwavelength in a ring waveguide. When a ring resonant wavelength becomescloser to a wavelength of a CW light, a number of circulations of the CWlight in the ring waveguide increases, and a heating value increases.Accordingly, there is a relationship as illustrated in FIG. 4B between aheating value according to absorption of a resonance light and a ringresonant wavelength. A heating value and a ring resonant wavelength in aring waveguide are therefore stabilized at any of three intersections21, 22, and 23 in FIG. 4C where both the relationship illustrated inFIG. 4A and the relationship illustrated in FIG. 4B are satisfied.

Note that the intersection 22 is an unstable point among the threeintersections. For example, when the ring resonant wavelength shiftstoward a long wavelength side from the intersection 22, a positivefeedback is applied, in which the heating value by the absorption of theresonance light increases, and it becomes further longer wavelength. Asa result, it is finally stabilized at the intersection 23. When the ringresonant wavelength shifts toward a short wavelength side from theintersection 22, a positive feedback is applied, in which the heatingvalue by the absorption of the resonance light decreases, and it becomesfurther shorter wavelength. As a result, it is finally stabilized at theintersection 21. When the ring resonant wavelength is at the longwavelength side from the intersection 22, it stabilizes at theintersection 23, and when the ring resonant wavelength is at the shortwavelength side from the intersection 22, it stabilizes at theintersection 21, as represented by arrows on a straight line in FIG. 4C,even when a wider wavelength range is considered without being limitedto a periphery of the intersection 22.

Accordingly, as illustrated in FIG. 4C, a phenomenon as described belowoccurs in a case where a heating value in a first relationship is largerthan a heating value in a second relationship at a ring resonantwavelength (wavelength of CW light) giving a maximum to the firstrelationship, where the first relationship is a relationship between aring resonant wavelength and a heating value according to absorption ofa resonance light in the ring waveguide, and the second relationship isa relationship between a heating value and a ring resonant wavelengthwhich changes according to the heating value in the ring waveguide. Inother words, if a ring modulator having a relationship as illustrated inFIG. 5A in an initial stage is forced to be heated with a heater inorder to set a ring resonant wavelength at a long wavelength side fromthe intersection 22 as illustrated in FIG. 5B, and thereafter theheating is stopped, the ring resonant wavelength is stabilized at theintersection 23, as illustrated in FIG. 5C. Once it is stabilized asstated above, the ring resonant wavelength is locked at the intersection23 even if the heater is not operated, and therefore, the powerconsumption of the heater can be set to be zero. Once it is locked, itis automatically continued to be locked, and therefore, observation ofan emitted light, a feedback control, and so on are not necessary.

Note that in an example illustrated in FIG. 5B, the heating is performeduntil the ring resonant wavelength becomes the longer wavelength thanthe intersection 23, but the heating may be performed such that the ringresonant wavelength becomes the longer wavelength than the intersection22. Namely, the heating may be performed up to a temperature exceeding atemperature corresponding to a nearest one (intersection 22) at theshort wavelength side from the ring resonant wavelength (wavelength ofCW light) giving the maximum to the first relationship among theintersections 21 to 23 between the graphic chart (FIG. 4B) representingthe first relationship and the graphic chart (FIG. 4A) representing thesecond relationship.

The wavelength at the stabilization point as stated above is awavelength suitable for applying an on/off modulation for the CW light.Hereinafter, that is described. In the ring modulator, as illustrated inFIG. 6, a CW light is modulated by changing the modulation voltage Vbetween the voltage V_(low) and the voltage V_(high) to thereby shiftthe ring resonant wavelength. The heating value according to theabsorption of the resonance light changes in accordance with themodulation voltage V as illustrated in FIG. 7A. The ring resonantwavelength during modulating changes from moment to moment, andtherefore, it is not defined uniquely, but in FIG. 7A, it is defined asthe ring resonant wavelength when the modulation voltage V is thevoltage V_(low) for convenience. As illustrated in FIG. 7A, curves eachrepresenting the heating value caused by the absorption of the resonancelight change depending on a case where the modulation voltage V is thevoltage V_(low) and a case where the modulation voltage V is the voltageV_(high). A temperature of the ring waveguide changes slowly in reactionto switching of the modulation voltage V. Accordingly, when a mark ratiois 50%, time-averages of these two curves corresponds to a curve 31representing the heating value according to the resonance lightabsorption. It is possible to stabilize the ring resonant wavelength atthe intersection 23 by shifting the ring resonant wavelength toward thelong wavelength side from the intersection 22 with the heater, andthereafter stopping the heating, even when the modulation is applied asstated above, as illustrated in FIG. 7B as same as the relationshiprepresented in FIG. 5A to FIG. 5C. At this time, the wavelength of CWlight is at a position illustrated in FIG. 8 relative to a transmissionspectrum, and the ring resonant wavelength is locked at a wavelengthcapable of applying modulation between the transmittance of the voltageV_(low) and the transmittance of the voltage V_(high).

Note that it is possible to continue to lock a ring resonant wavelengthat a modulated light also for a burst signal in which a state without asignal (burst-off state) and a state with a signal (burst-on state) aremixed. Namely, as illustrated in FIG. 9, it is possible to continue tolock a ring resonant wavelength at a modulated light also for a burstsignal in which a state in which the modulation voltage V is constantlythe voltage V_(low) (burst-off state) and a state in which themodulation voltage V is switched at random between the voltage V_(low)and the voltage V_(high) (burst-on state) are mixed. In FIG. 9, a casewhere the mark ratio of the burst-on state is 50%. Intersections betweena curve 33 and a straight line 32 under the burst-off state aredifferent from the intersections 22, 23 between the curve 31 and thestraight line 32 under the burst-on state. Here, the intersections underthe burst-off state corresponding to the intersections 22, 23 aredescribed as 22′, 23′.

When a state is switched into the burst-off state when the ring resonantwavelength is locked at the intersection 23 under the burst-on state,the intersection 23 comes to be not a stable state. Note that theintersection 23′ positions at the long wavelength side from theintersection 22′, and therefore, it is finally stabilized at theintersection 23′. When the state is switched into the burst-on stateunder a state when the ring resonant wavelength is stabilized at theintersection 23′, the intersection 23′ similarly comes to be not thestable state, but it positions at the long wavelength side than theintersection 22, and therefore, the ring resonant wavelength isstabilized at the intersection 23. As stated above, when the burst-onstate and the burst-off state are switched, the ring resonant wavelengthtransits between the intersection 23 and the intersection 23′. Even ifthe transition as stated above occurs, the lock is not disengaged, andit is possible to return to the intersection 23 when the state becomesthe burst-on state.

Second Embodiment

Next, a second embodiment is described. FIG. 10A is a view illustratinga layout of an optical semiconductor element according to the secondembodiment, FIG. 10B is a sectional view along a I-II line in FIG. 10A,and FIG. 10C is a sectional view along a III-II line in FIG. 10A.

In the second embodiment, as illustrated in FIG. 10B and FIG. 10C, anSiO₂ film 102 is formed on an Si substrate 101. As illustrated in FIG.10A to FIG. 10C, an n⁺ layer 105 n, an n⁻ layer 104 n, a p⁻ layer 104 pand a p⁺ layer 105 p each in a ring shape are formed in this sequencearranged from inside on the SiO₂ film 102. For example, impurity dopedSi is used for each of the n⁺ layer 105 n, the n⁻ layer 104 n, the p⁻layer 104 p and the p⁺ layer 105 p. An SiO₂ film 106 covering the n⁺layer 105 n, the n⁻ layer 104 n, the p⁻ layer 104 p and the p⁺ layer 105p is formed on the SiO₂ film 102, and a ring-shaped heater 107 is formedabove the n⁻ layer 104 n and the p⁻ layer 104 p on the SiO₂ film 106.The SiO₂ film 106 functions as a cladding layer. An SiO₂ film 108covering the heater 107 is formed on the SiO₂ film 106. A hole reachingthe n⁺ layer 105 n and a hole reaching the p⁺ layer 105 p are formed inthe SiO₂ film 108 and the SiO₂ film 106. A modulating electrode 110 nconnected to the n⁺ layer 105 n through the hole reaching the n⁺ layer105 n, and a modulating electrode 110 p connected to the p⁺ layer 105 pthrough the hole reaching the p⁺ layer 105 p are formed on the SiO₂ film108. An SiO₂ film 111 covering the modulating electrodes 110 n and 110 pis formed on the SiO₂ film 108. A ring waveguide may be composed asstated above. Note that in FIG. 10A, the SiO₂ films 102, 106, 108 and111 are represented as an SiO₂ film 125 as a whole.

A straight waveguide 121 where the light comes and goes to/from the ringwaveguide is formed in the vicinity of the ring waveguide. A nearlyring-shaped state light absorbing material 114 a is provided at aperiphery of the ring waveguide apart from a path of a light between thering waveguide and the waveguide 121. The light absorbing material 114 ais formed from a position below a surface of the Si substrate 101 toreach a surface of the SiO₂ film 111. A groove 115 is formed at aperiphery of the light absorbing material 114 a, and a groove 122 isformed on the opposite side of the waveguide 121 from the ringwaveguide.

Two electrodes 124 making a current flow in the heater 107 are provided.A predetermined voltage is applied between the two electrodes 124, andthereby, the current flows in the heater 107, and Joule heat isgenerated. For example, Ti is used for the heater 107. W, Pt, or dopedSi may be used for the heater 107. A material is not limited to theabove as long as it is a material capable of forming a stable highresistant film. Two electrodes 123 each connected to the modulatingelectrodes 110 n, 110 p are also provided.

For example, an ultraviolet (UV) cure polymer or a thermosetting polymercontaining a dye absorbing an incident light wavelength is used for thelight absorbing material 114 a. A single crystal, polycrystal, oramorphous Si or Ge may be used for the light absorbing material 114 a,and one in which light absorption is enhanced by high concentrationdoping thereto may be used. The material is not limited thereto as longas it is a material having a strong absorptivity for the modulated lightwavelength.

In the optical semiconductor element constituted as stated above, a sizeof a depletion region of a pn junction between the n⁻ layer 104 n andthe p⁻ layer 104 p of the ring waveguide changes in accordance with themodulation voltage V, and a modulation of a refractive index of awaveguide mode 130 is enabled according to this change. A light 131leaks out of the ring waveguide, but a part thereof is absorbed by thelight absorbing material 114 a, a heat generation 132 occurs at thelight absorbing material 114 a absorbing the light, and the temperatureof the ring waveguide is increased. Accordingly, it is possible to lockthe ring resonant wavelength by heating the ring waveguide with theheater 107 before the modulation operation is started to thereby makethe ring resonant wavelength higher than a wavelength similar to theintersection 22 in the first embodiment, and thereafter, the heatingwith the heater 107 is stopped.

Accordingly, it is possible to enable a stable control. Furthermore, itis not necessary to continue to operate the heater 107, and therefore,it is possible to suppress the power consumption low.

The grooves 115 and 122 are formed, and therefore, a heat resistance ofthe grooves 115, 122 with outside increases to thereby reduce the heatloss. It is thereby possible to effectively utilize the temperatureincrease with the heater 107 and the temperature increase by the lightabsorption for the temperature increase of the ring waveguide.

Third Embodiment

Next, a third embodiment is described. FIG. 11A is a view illustrating alayout of an optical semiconductor element according to the thirdembodiment, FIG. 11B is a sectional view along a I-II line in FIG. 11A,and FIG. 11C is a sectional view along a III-II line in FIG. 11A.

In the third embodiment, as illustrated in FIG. 11A to FIG. 11C, a ringstate light absorbing material 114 b is provided at inside of the n⁺layer 105 n, and the groove 115 is provided also at inside of the ringstate light absorbing material 114 b. The other configuration is similarto the second embodiment.

In the third embodiment as stated above, the absorption of the light 131and the heat generation 132 occur also in the light absorbing material114 b. Accordingly, it is possible to use the heat more efficiently thanthe second embodiment.

Note that the light absorbing material 114 a is not necessarily providedif the light absorbing material 114 b is provided.

Fourth Embodiment

Next, a fourth embodiment is described. FIG. 12A is a view illustratinga layout of an optical semiconductor element according to the fourthembodiment, FIG. 12B is a sectional view along a I-II line in FIG. 12A,and FIG. 12C is a sectional view along a III-II line in FIG. 12A.

In the fourth embodiment, as illustrated in FIG. 12A to FIG. 12C, a ringstate light absorbing material 114 c is formed on the SiO₂ film 111 soas to connect the light absorbing materials 114 a and 114 b.Accordingly, almost all of the ring waveguide is covered from above withthe light absorbing material 114 c. The other configuration is similarto the third embodiment.

In the fourth embodiment as stated above, the absorption of the light131 and the heat generation 132 occur also in the light absorbingmaterial 114 c. Accordingly, it is possible to use the heat further moreefficiently than the third embodiment.

Note that one of or both of the light absorbing materials 114 a and 114b is/are not necessarily provided if the light absorbing material 114 cis provided.

Fifth Embodiment

Next, a fifth embodiment is described. FIG. 13A is a view illustrating alayout of an optical semiconductor element according to the fifthembodiment, FIG. 13B is a sectional view along a I-II line in FIG. 13A,and FIG. 13C is a sectional view along a III-II line in FIG. 13A.

In the fifth embodiment, as illustrated in FIG. 13A to FIG. 13C, a ringstate light absorbing material 114 d is formed under the SiO₂ film 102so as to connect the light absorbing materials 114 a and 114 b.Accordingly, almost all of the ring waveguide is covered from underneathwith the light absorbing material 114 d. A hollow part 116 exists underthe light absorbing material 114 d. The other configuration is similarto the fourth embodiment.

In the fifth embodiment as stated above, the absorption of the light 131and the heat generation 132 occur also in the light absorbing material114 d. In the fifth embodiment, almost all of the ring waveguide issurrounded by the light absorbing materials 114 a, 114 b, 114 c and 114d from all directions. Accordingly, it is possible to further moreefficiently use the heat than the fourth embodiment, and the heat lossseldom occurs. Further, the existence of the hollow part 116 largelycontributes to the reduction of the heat loss.

Note that the light absorbing materials 114 a, 114 b or 114 c or anycombination thereof is/are not necessarily provided if the lightabsorbing material 114 d is provided.

Sixth Embodiment

Next, a sixth embodiment is described. The sixth embodiment relates to amethod of controlling the optical semiconductor elements according tothe first to fifth embodiments. FIG. 14A is a flowchart illustrating anexample of a method of controlling the optical semiconductor elementaccording to the sixth embodiment, and FIG. 14B is a flowchartillustrating another example.

In the method illustrated in FIG. 14A, the heater 107 is operated (stepS12) from an initial stage (step S11). Then, the multi-wavelength lightsource is also operated (step S13), and thereafter, the operation of theheater 107 is stopped (step S14) by stopping the application of thevoltage to the heater 107. Then the modulation operation is started(step S15).

In the method illustrated in FIG. 14B, the multi-wavelength light sourceis operated (step S22) from an initial stage (step S21). Then, theheater 107 is also operated (step S23), and thereafter, the operation ofthe heater 107 is stopped (step S24) by stopping the application of thevoltage to the heater 107. Then the modulation operation is started(step S25).

The controls as stated above are performed, and thereby, it is possibleto lock the ring resonant wavelength according to any of the examples.

Note that a ring resonant wavelength at an initial stage is set to be awavelength shorter than the modulated light wavelength. Namely, a ringresonant wavelength at an initial stage may vary depending onmanufacturing accuracy and a temperature, and therefore, a ring resonantwavelength at an initial stage is set so that the ring resonantwavelength is shorter than the modulated light wavelength even in a casewhere the ring resonant wavelength at the initial stage varies on thelongest wavelength side. Besides, a voltage V_(heater, on) of the heater107 during the operation is set so that the ring resonant wavelengthduring the operation is longer than the modulated light wavelength evenin a case where the ring resonant wavelength at the initial stage varieson the shortest wavelength side.

Seventh Embodiment

Next, a seventh embodiment is described. The seventh embodiment relatesto a method of manufacturing an optical semiconductor element similar tothe fifth embodiment. FIG. 15A to FIG. 15P are sectional viewsillustrating the method of manufacturing the optical semiconductorelement according to the seventh embodiment in process sequence.

First, as illustrated in FIG. 15A, an SiO₂ film 202 is formed on an Sisubstrate 201, and an Si film 203 is formed thereon. Then, asillustrated in FIG. 15B, a mesa part 203 a is formed at the Si film 203by etching the Si film 203 at a periphery of a region where an n⁺ layeris to be formed and a region where a p⁺ layer is to be formed.Thereafter, an n⁻ layer 204 n is formed by doping an n-type impurity atlow concentration into the region where the n⁺ layer is to be formed anda region where an n⁻ layer is to be formed, and a p⁻ layer 204 p isformed by doping a p-type impurity at low concentration into the regionwhere the p⁺ layer is to be formed and a region where a p⁻ layer is tobe formed. Either of the n⁻ layer 204 n or the p⁻ layer 204 p may beformed first. Subsequently, as illustrated in FIG. 15D, an n⁺ layer 205n is formed by further doping an n-type impurity into the region wherethe n⁺ layer is to be formed, and a p⁺ layer 205 p is formed by furtherdoping a p-type impurity into the region where the p⁺ layer is to beformed. Either of the n⁺ layer 205 n or the p⁺ layer 205 p may be formedfirst. Then, as illustrated in FIG. 15E, a part of the n⁺ layer 205 nand a part of the p⁺ layer 205 p is etched so as to remain the n⁺ layer205 n and the p⁺ layer 205 p at a region where the ring waveguide is tobe formed.

Thereafter, as illustrated in FIG. 15F, an SiO₂ film 206 covering the n⁺layer 205 n, the n⁻ layer 204 n, the p⁻ layer 204 p and the p⁺ layer 205p is formed on the SiO₂ film 202. The SiO₂ film 206 functions as acladding layer. Subsequently, as illustrated in FIG. 15G, a ring stateheater 207 is formed at a position above the n⁻ layer 204 n and the p⁻layer 204 p on the SiO₂ film 206. Then, as illustrated in FIG. 15H, anSiO₂ film 208 covering the heater 207 is formed on the SiO₂ film 206.

Thereafter, as illustrated in FIG. 15I, a hole 209 n reaching the n⁺layer 205 n and a hole 209 p reaching the p⁺ layer 205 p are formed inthe SiO₂ film 208 and the SiO₂ film 206. Subsequently, as illustrated inFIG. 15J, a modulating electrode 210 n connected to the n⁺ layer 205 nthrough the hole 209 n and a modulating electrode 210 p connected to thep⁺ layer 205 p through the hole 209 p are formed on the SiO₂ film 208.Then, as illustrated in FIG. 15K, an SiO₂ film 211 covering themodulating electrode 210 n and the modulating electrode 210 p is formedon the SiO₂ film 208.

Thereafter, as illustrated in FIG. 15L, grooves 212 are formed atregions where light absorbing materials are to be formed, namely, atinside and outside of the ring waveguide. A depth of the groove 212 isset to be, for example, a degree reaching inside of the Si substrate201. Subsequently, as illustrated in FIG. 15M, a hollow part 213connecting the grooves 212 with each other is formed by isotropicetching a surface layer part of the Si substrate 201 through the grooves212. Then, as illustrated in FIG. 15N, a light absorbing material 214 isformed inside of the hollow part 213 and inside of the grooves 212. Atthis time, the light absorbing material 214 is formed to connect thegrooves 212 with each other also above the SiO₂ film 211.

Thereafter, as illustrated in FIG. 15O, grooves 215 are formed on bothsides of the light absorbing material 214. At this time, a position of abottom of the groove 215 is set to be deeper than a bottom of the lightabsorbing material 214. Subsequently, as illustrated in FIG. 15P, ahollow part 216 connecting the grooves 215 with each other is formed byisotropic etching the surface layer part of the Si substrate 201 throughthe grooves 215.

Thus, the optical semiconductor element similar to the fifth embodimentmay be manufactured. When an optical semiconductor element similar tothe fourth embodiment is manufactured, the formation of the hollow part213 is omitted so that the light absorbing material 214 does not enterinto a lower part of the ring waveguide, and further, the formations ofthe grooves 215 and the hollow part 216 are omitted. When an opticalsemiconductor element similar to the third embodiment is manufactured,further, the light absorbing material 214 is not to be formed on theSiO₂ film 211. For example, a mask may be formed in advance. When anoptical semiconductor element similar to the second embodiment ismanufactured, further, the formation of the groove 212 inside of thering waveguide is omitted, and the groove 212 is formed only outside ofthe ring waveguide.

Eighth Embodiment

Next, an eighth embodiment is described. FIG. 16A is a view illustratinga structure of an optical semiconductor element according to the eighthembodiment.

In the eighth embodiment, as illustrated in FIG. 16A, N-pieces of ringmodulators 41 ₁ to 41 _(N) similar to the fifth embodiment are providedalong the waveguide 1. Namely, the N-pieces of ring modulators 41 ₁ to41 _(N) are cascade-connected. Ring circumferential optical path lengthsof the ring modulators 41 ₁ to 41 _(N) are different from one another,and as illustrated in FIG. 16B, respective ring resonant wavelengths ofthe ring modulators 41 ₁ to 41 _(N) at an initial stage are λ₁′, λ₂′, .. . , λ_(N)′. A multi-wavelength light source 42 is connected to aninput of the waveguide 1, and a multi-wavelength light having N-kinds ofoscillation wavelengths λ₁, λ₂, . . . , λ_(N) is input from themulti-wavelength light source 42 to the waveguide 1. Here, asillustrated in FIG. 16B, the oscillation wavelengths λ₁, λ₂, . . . ,λ_(N) are longer than the ring resonant wavelengths λ₁′. λ₂′, . . . ,λ_(N)′ respectively at the initial stage. The oscillation wavelengthsλ₁, λ₂, . . . , λ_(N) and the ring resonant wavelengths λ₁′, λ₂′, . . ., λ_(N)′ are set so that the ring resonant wavelengths λ₁′, λ₂′, . . . ,λ_(N)′ are longer than the oscillation wavelengths λ₁, λ₂, . . . , λ_(N)when the respective heaters of the ring modulators 41 ₁ to 41 _(N) areoperated.

When the respective heaters are operated for the eighth embodiment asstated above as same as the sixth embodiment, the ring resonantwavelengths λ₁′, λ₂′, . . . , λ_(N)′ become longer than the oscillationwavelengths λ₁, λ₂, . . . , λ_(N) as illustrated in FIG. 16B. Once theoperation of the respective heaters are stopped under this state as sameas the sixth embodiment, the ring resonant wavelengths λ₁′, λ₂′, . . . ,λ_(N)′ are respectively locked at the wavelengths suitable for themodulation of the oscillation wavelengths λ₁, λ₂, . . . , λ_(N) asillustrated in FIG. 16B. It becomes possible to apply the appropriatemodulation on this wavelength.

In this embodiment, the ring resonant wavelengths λ₁′, λ₂′, . . . ,λ_(N)′ of the ring modulators 41 ₁ to 41 _(N) are set so that, thefarther from the input (light source) and the closer to the output, thelonger the ring resonant wavelength is, namely, a relationship of“λ₁′<λ₂′< . . . , <λ_(N)′” is satisfied. A sequence of an order of thering resonant wavelengths λ₁′, λ₂′, . . . , λ_(N)′ is not limitedthereto, and the ring modulators 41 ₁ to 41 _(N) may be arranged in anysequence.

Though the ring modulators 41 ₁ to 41 _(N) similar to the fifthembodiment are provided in the present embodiment, the ring modulators41 ₁ to 41 _(N) may have structures similar to any one of the first tofourth embodiments. An example of controlling in which the ring resonantwavelength intersects with only one oscillation wavelength during theheater operation is illustrated in FIG. 16B, two or more oscillationwavelengths may be intersected.

Ninth Embodiment

Next, a ninth embodiment is described. The ninth embodiment relates toan optical transmitting and receiving device. FIG. 17 is a viewillustrating an optical semiconductor element according to the ninthembodiment.

A multi-wavelength light source 52 emitting a multi-wavelength lighthaving eight kinds of oscillation wavelengths λ₁, λ₂, . . . , λ₈ isprovided to an optical transmitting and receiving device 51 accordingthe ninth embodiment, and waveguides 59 are connected to themulti-wavelength light source 52. A ring modulation group 50 includingeight pieces of ring modulators is provided by each waveguide 59. Ringresonant wavelengths of the eight pieces of ring modulators at aninitial stage are λ₁′, λ₂′, . . . , λ₈′. Further, a driver for modulator53 controlling modulation of each ring modulator is provided. The driverfor modulator 53 controls, for example, a modulation voltage of eachring modulator. An output port 57 is provided at each waveguide 59.

Further, input ports 58 are provided to the optical transmitting andreceiving device 51, and for example, a demultiplexer 56 in a wavelengthdivision multiplexing system (WDM) is connected to each input port 58. Aphotodiode 55 receiving an optical signal output from the demultiplexer56 is provided, and a TIA (trace impedance amplifier)/LA (limitingamplifier) 54 inputting an output of the photodiode 55 is provided.

In the ninth embodiment as stated above, the ring modulator group 50operates similarly to the eighth embodiment. Accordingly, the ringresonant wavelengths λ₁′, . . . , λ₈′ are respectively locked at thewavelengths suitable for the modulation of the oscillation wavelengthsλ₁, . . . , λ₈, and it is possible to apply the appropriate modulationat the wavelengths.

It should be noted that the above embodiments merely illustrate concreteexamples of implementing the present invention, and the technical scopeof the present invention is not to be construed in a restrictive mannerby these embodiments. That is, the present invention may be implementedin various forms without departing from the technical spirit or mainfeatures thereof.

According to the optical semiconductor element and so on, it is possibleto stably control a ring resonant wavelength to be one suitable formodulation owing to an operation of the light absorbing material.

All examples and conditional language provided herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventor to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent invention have been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

According to the above-stated optical semiconductor elements and so on,it is possible to stably control the ring resonant wavelengths suitablefor the modulation owing to the operations of the light absorbingmaterials.

What is claimed is:
 1. An optical semiconductor element, comprising:ring modulators whose circumferential optical path lengths are differentfrom one another; and light absorbing materials, each of the lightabsorbing materials provided at a position apart from a path for amodulated light which guided by each of the ring modulators, absorbing alight leaked out of a ring waveguide of each of the ring modulators, andincreasing a temperature of the ring waveguide, wherein a heating valuein a first relationship is larger than a heating value in a secondrelationship at a ring resonant wavelength giving a maximum to the firstrelationship, the first relationship being a relationship between a ringresonant wavelength and a heating value according to absorption of aresonance light in the ring waveguide, and the second relationship beinga relationship between a heating value and a ring resonant wavelengthwhich changes according to the heating value in the ring waveguide.
 2. Amethod of manufacturing an optical semiconductor element, the methodcomprising: forming a ring modulator; and forming a light absorbingmaterial at a position apart from a path for a modulated light which isguided by the ring modulator, the light absorbing material absorbing alight leaked out of a ring waveguide of the ring modulator, andincreasing a temperature of the ring waveguide, wherein a heating valuein a first relationship is larger than a heating value in a secondrelationship at a ring resonant wavelength giving a maximum to the firstrelationship, the first relationship being a relationship between a ringresonant wavelength and a heating value according to absorption of aresonance light in the ring waveguide, and the second relationship beinga relationship between a heating value and a ring resonant wavelengthwhich changes according to the heating value in the ring waveguide.